Hearing aid transistor amplifiers



March 1, 1960 HERRMANN ET AL 2,927,275

HEARING AID TRANSISTOR AMPLIFIERS Filed April 13, 1956 United StatesPatent HEARING AID TRANSISTOR AMPLIFIERS Frank A. Herrmann, WhitePlains, N.Y., and Debdas Chaudhuri, Burdwan, West Bengal, India,assignors to Sonotone Corporation, Elmsford, N.Y., a corporation of NewYork Application April 13, 1956, Serial No. 584,159

4 Claims. (Cl. 330-17) (Filed under Rule 47(a) and 35 U.S.C. 116) Thisinvention relates to amplifiers, and particularly to hearing aidtransistor amplifiers of the type used in tiny, compact, miniaturehearing aid amplifier units worn inconspicuously on the body of theuser, although features of the invention have application also in othertransistor amplifiers.

Among the objects of the invention is a miniature transistor amplifierrequiring a considerably smaller number of circuit components than inknown prior hearing-aid transistor amplifiers, and which operate withhigh efficiency and stability over a wide range of variations in thecharacteristics of the transistors and other components due totemperature changes.

The foregoing and other objects of the invention will be best understoodfrom the following description of exemplifications thereof, referencebeing had to the accompanying drawings, wherein:

Fig. 1 is a circuit diagram of one practical form of a miniature hearingaid amplifier exemplifying the invention; and

Fig. 2 is a similar circuit diagram of another exemplification of theinvention.

In any transistor amplifier circuit, it is essential to limit theemitter and collector currents so that the voltage difierence from theemitter to the collector shall not be less than the minimum valuerequired for eifective amplification. When the energy supply voltage islimited, such as in cases when the energy supply voltage is only thatcorresponding to one or two battery cells, too much current will resultin a voltage drop in the resistance of the emitter-collector circuitwhich will be sufiicient to lector to below such minimum value. Anincrease in "ice its operation over wide variations of temperature andhumidity while worn on the body of the user.

Fig. 1 shows the circuit diagram of one form of tiny miniaturehearing-aid amplifier of the invention occupying a volume of only 1.5cubic inches. The hearing-aid amplifier shown has four successivetransistor amplifier stages operating with transistors -11, 12, 13 and14, for amplifying the output of a hearing-aid microphone 15 anddelivering it to a receiver 16. All elements of the amplifier are housedin the miniature amplifier casing indicated by dash-double-dot line 10,except for the receiver 16, which may be either a miniature earphone worn hidden in the ear of the user, or a miniature bone receiver wornhidden in contact with a hearing-inducing bone of the user. stages areenergized from the same power supply source 17 shown in the form of asingle battery cell, the two poles of which are connected through anon-and-oii switch 18 to two supply leads 21, 22 of opposite polarity. Tosimplify the description, supply lead 21 which corresponds to what isknown as a ground conductor or lead, will be designated as thelow-potential lead, and the other supply lead 22, as the high-potentiallead. It should be noted that amplifiers of the invention may be readilydesigned to operate with power supplies of much higher voltages, such asused in other applications.

In the amplifier of Fig. 1, each of the transistors 11, 12, 13 and 14 isconnected for operation with a common emitter configuration. Inaccordance with accepted practice, the term common emitter configurationof a transistor circuit, means that the input is impressed on the baseand emitter, and that the output is delivered by the collector andemitter of the transistor. The microphone 15 is of the electromagnetictype, and sound generates in its coil winding a signal current which isdelivered through leads 25, 26 to the base and the emitter of transistor11 of the first amplifier stage. Direct-current biasing for transistor11 and also some of the other transistors, is provided byvoltage-dividing resistors or bias resistance elements 27, 28 which areconnected across the energy supply leads 21, 22 and have an intermediateresistor portion or resistor connection 29 to which lead 26 from thewinding of the microphone 15 is connected for supplying therethrough theproper bias current to the base of transistor 11. The collector oftransistor 11 is directly connected through lead 31 to thehigher-potential supply lead 22. The emitter of transistor 11 isconnected through lead 32 having connected therein a load rcsistthetransistor temperature, particularly in the case of transistor circuitsoperating with a common emitter configuration, is accompanied by a largeincrease in emitter current unless the bias of the transistor iscontrolled to minimize such undesirable current increase due totemperature.

In miniature transistor hearing aid amplifiers, the individualtransistors, resistances, capacitors and other circuit components mustbe compressed into an extremely small volume. All these components mustbe of the minutes't possible size, and they must operate with highstability and relatively long useful service life under severe operatingconditions on the body of the user over a wide range of temperature andhumidity conditions. The present invention makes it possible to reducethe number of the miniature components required to provide a highlyefficient miniature transistor hearing-aid amplifier .having at leasttwo successive transistor amplifier stages operating with a commonemitter configuration which are biased to minimize undesirable increasein emitter current and which will operate with the desired overallfrequency response. It also simplifies the amplifier circuits, andthereby simplifies the control of the critical factors which determinethe reliability and stability of ance 33 to the lower-potential supplyline 21, thereby completing the biasing and operating connections of thetransistor 11, the operation of which will be explained hereinafter. Thesignal input circuit from microphone winding 15 to the base and emitterof transistor 11 is completed by by-pass capacitor 34. This capacitor 34also permits the application of direct-current bias to the base whileproviding a low impedance signal path to the emitter of transistor 11.The output signal current of transistor 11 flows from its collector,through collector lead 31, supply lead 22, switch 18, battery 17, supplylead 21, back through emitter lead 32 and load resistance 33 to theemitter, the output being developed across load resistance 33.

In accordance with a phase of. the invention disclosed herein, thebiasing circuit elements including the voltage-' dividing resistanceelements 27, 28 of transistor ll of this amplifier stage are sodesigned'and arranged as to also provide at the same intermediateresistorportionor connection 29, a common direct-current biasing connection not only for the base of transistor 11, but also for the base'ofthe next transistorl12 ofthe next'transistor" stage, through thedirect-currentleadiSS thereto, through which also the signal outputdeveloped across load re The circuits of all four transistor amplifierpressed on the capacitor at an intermediate setting of the volumecontrol resistance 45.

With the above-described arrangement, when the volume-control tap 45-1is in the maximum-volume position and provides a direct connection tothe supply lead 21, capacitor 46 by-passes the emitter resistance 42 oftransistor 13, and its degenerative action is suppressed. In the otherextreme position of the movable volumecontrol tap 45-1, by-passcapacitor 46 acts as a short circuit for signal currents between thebase and the emitter of transistor 13. When the movable volume controltap 45-1 is moved from the maximum-volume position to lower-volumepositions, it reduces the signal by-passing action of capacitor 46across the emitter lead resistance 42, gradually increasing itsdegenerative action, and in addition capacitor 46 starts to by-pass thesignals impressed between the base and emitter of transistor 13 until inthe minimum-volume position, capacitor 46 provides a short circuit forsignals between the base and the emitter of transistor 13.

The transistor 13 is provided with proper automatic bias control by thevoltage-dividing resistance network of the preceding transistor 12consisting of emitter resistor 37, the efiective internal direct-currentresistance between the collector and emitter, and the collector loadresistance 39 of the preceding transistor 12. The con nection of thebase of transistor 13 to an intermediate portion of thisvoltage-dividing network-atv the collector of the preceding transistor12together with the resistance 42 in the emitter lead of transistor 13,provide automatic bias control in the same manner as explained above inconnection with the bias control of transistors 11, 12. Thus an increasein the emitter current of transistor 13, due to temperature, forinstance, increases the voltage drop across emitter resistance 42,thereby decreasing the voltage difference between its emitter and base,thus minimizing the increase of emitter current that would otherwiseoccur.

The amplified signal developed by the transistor 13 across its collectorload resistance 44 is delivered to the base of the last amplifier stagetransistor 14 through one or both parallel-connected coupling capacitors51, 51-1, a cut-out switch 52 making it possible to cut out one of thecapacitors so as to reduce the low-frequency response in accordance withthe requirements of the hardof-hearing person using it.

In accordance with a phase of the invention disclosed herein, automaticbias control for a transistor of a multitransistor amplifier is securedby taking advantage of the fact that all amplifier transistors arehoused in the same compact casing, and are subjected to substantiallyequal temperature changes which will cause all their emitter currents toincrease with increase in temperature. In accordance with the invention,the bias applied to the electrodes of transistor 14 is automaticallycontrolled by the increase of current in a preceding transistor due totemperature change, for modifying the bias applied to the succeedingtransistor 14 so as to minimize the change of current that wouldotherwise occur. To this end, in the particular amplifier of Fig. 1, thebase of the transistor 14 is connected through lead 54, which includesthe resistance 55, to the junction between the emitter of transistor 13and resistance 42 through which this emitter is connected to the energysupply. The tendency of the emitter current of transistor 14 to increasedue to temperature will be accompanied by a similar increase of theemitter current of transistor 13, which will produce an increasedvoltage drop across resistance 42 in the emitter lead of transistor 13.The increased voltage drop across bias resistance 42 of transistor 13reduces the voltage available for biasing the base of transistor 14through lead 54 and its resistance 55, thereby minimizing the increaseof emitter current of transistor 14 that would otherwise occur due toincrease of temperature.

Resistors Resistor Kilo-Ohms 27 10 9X 5. 6 qq 1. 5 37 1. 5 an 1. 5 42 1.2 44 1. 5 4s 7. 5 5 10 to Capacitors Capacitor Mlcroiarads Type VoltageRating 4 Electrolytic 4 16 ...do 1 16 1 1 4 03 Ceramic The value ofresistance 54 is chosen to match transistor 14. The winding ofmicrophone 15 has at 1000 c.p.s., resistance of 1500 ohms. The windingof receiver 16 has at 1000 c.p.s., an impedance of 550 ohms, and adirectcurrent resistance of 220 ohms.

In cases where only one type of transistor, for instance only PNPtransistors or only NPN transistors are available for use in amplifiersof the type described above, transistor 12 of the second amplifier stagehas to be connected to the same type of transistor as the nextamplifying stage through a coupling capacitor. Fig. 2 shows suchmodified form of hearing-aid amplifier of the invention, consisting ofthe same combination of elements except for the modifications describedbelow. The third transistor amplifier stage operates with the same typeof transistor 13-1 as the three other transistors 11, 12 and 14 of thisamplifier. The output developed across the load resistance 39 of thepreceding transistor 12 is delivered to the base of the next transistor123-1 through a coupling capacitor 61. The same type of volume control45 with a movable contact tap 45-1 is connected through a couplingcapacitor 46 to the emitter of transistor 13-1. The base of transistor131 is supplied with the proper bias by connecting across the supplyleads 21, 22 another set of bias resistance elements 63, 64, having anintermediate resistance portion or connection 66 to which the base oftransistor 13-1 is connected. Otherwise, the amplifier of Fig. 2 isidentical with the amplifier of Fig. l.

The features and principles underlying the invention described above inconnection with specific exemplifications will suggest to those skilledin the art many other modifications thereof. It is accordingly desiredthat the appended claims shall not be limited to any specific feature ordetails thereof.

We claim:

1. In an amplifier for a device, such as a hearing aid worn on the bodyof the user, having at least three successive amplifier stages energizedby a common energy supply having two opposite supply poles, a firstamplifier stage having a first transistor for amplifying signals of asignal source applied to the base thereof, a second amplifier stagehaving a second transistor connected to the output side of said firsttransistor for amplifying the signal output of the first transistor, athird amplifier stage an impedance of 5000 ohms, and adirect-currenthaving athird transistor connected to the output side ofthefsecondrtran'sistor for further amplifying the signal oiitputof the.secondtransistor," theconecmr electrodesfof the first and secondtransistors "and the emitter electrode of the third transistorhaviiig"'chch ,a direct cur'rentconemento-ens of said two poles, and theer'fiitter electrodes of th e'jfi'rst and second'transis'to fs andthecollector elect'rbdeo'fme third transistor having each a directcurrentconnection to the other of said twopoles,jane1nit'-" Vter resistanceconnected between the emitterof each of said three-transistors andthe-supplypole of therespective emitter, the circuits of each ofsaid-three transistors having a common emitter configuration, a loadresistance connected between thecollectorrof said second transistor andits supply pole,said-outputsideof said secondtransistor including: adirect-current loadcon'riecftionj from the collector of the'second"transistor to the base of the third through the emitter of eitherone ofthe first and second transistors to "reduce the "bias applied throughthe common biasing connectio rniriiniizejsucwdi" H t; increase, and'anoutput impedanceinthe'o'titpuf' said thirdtran'sistor.

21 Inan ariiplifi 'cl ed sistanceelein'entbeing' nnecte said 'coniriionfbiasin'g" c'onnectionibeing established at an intermediate portion"offs'aid bias resistance element.

transistor for? impressing 'the r'eonfith outputfdevelopeii across saidloadresistanceg-the movable carriers of the third transistor being ofapeiarity'ep'pos'ite totliafo'f the movable carriers oftlfe firstandsecond transistois'g'th'e" direct-current resistance between thecollector and emitter of the second transistor and theresistanceconnecting its" collector and emitter to their said'respective poles being proportionedto develop atthe collector of saidsecond transistor a bias applied through said loadconnec'tion to thebase of the: third transistor which causes 'anincrease of itsemitter-current toreduce the biasatlits b ase andIthere by minimizeits'said'curreiit increase,"a"bias resistance element connected to thesupply and in confinion biasing connection with the'bases of the'firstand second transistors through which a'comrnondirect-current bias isapplied to their bases, said bias resistance element and the emitterresistances of the first and second transistors being proportioned to'cause increase-in the direct current tr'ol resistance connected betweenthe collector of said second transistor and" said other pole, avariabletap of said volume' control"resistance being coupled to the emitter ofthe: third" trahsistor for" impressing" thereon the outputidevelope'dacr'sssa variable portion of said control OTHER REFERENCES}? Shea";VPrineiplesfof Transistoi'j Circuits, pages 102 a 164, Sept. 1s;19s3.

' July 30, 1 957 n to'i'tsT 1 1 base and thereby

